1. Field of the Invention
This invention relates to a signal generation system applied to a mobile communication device such as a portable wireless telephone or the like, and more particularly to an orthogonal signal generation system for generating carrier signals or RF (radio frequency) signals whose phases are different from each other by 90.degree. and which are used for orthogonal modulation/demodulation.
2. Description of the Related Art
In recent years, as represented by portable wireless telephones and cellular (radio) phones, mobile communication devices which can communicate in desired places are actively developed. For example, since this type of communication device is carried by a man or mounted in a car, it is desired to reduce the size and weight of the communication device. For this purpose, parts of the communication device are required to be formed in a monolithic IC form suitable for reduction in size and weight rather than in the form of conventional hybrid IC (integrated circuit). Further, since it is necessary to drive the portable wireless telephone by use of a battery, it is desired to develop an IC which is operated on a low voltage.
In the above-described mobile communication device, an orthogonal modulation/demodulation system for superposing a speech signal on two carriers whose phases are different from each other by 90.degree. and transmitting the speech signal is generally used as a communication system. In order to realize the orthogonal modulation system, it is necessary to create carrier signals whose phases are different from each other by 90.degree. from a local signal generated from a local oscillator at high s/n (signal to noise) ratio. In this invention, a system for generating two output signals such as the two carrier signals set in an orthogonal relation is called an orthogonal signal generation system.
In the circuit of the conventional orthogonal signal generation system, a resister for terminating to prevent reflection of the local signal is arranged at an input terminal inputting the local signal. Since the voltage amplitude of the output signal of the orthogonal signal generation circuit is determined by the power of the input signal, causing a problem that a sufficiently high voltage gain cannot be obtained when the resister for terminating is set to 50.OMEGA. or 70.OMEGA. for example. In principle, it is possible to increase the voltage gain by selectively setting the value of the terminal resistor, but in the GHz band, one of the values 50.OMEGA. and 75.OMEGA. can be generally selected because of the impedance of the transmission line.
In general, transistors are employed in the circuit of the orthogonal signal generation system. Assuming that two cascated emitter follower are employed in the circuit and input signals are transmitted via the transistors, potentials of the output signals from the circuit are set to a low value by a voltage drop between a base and an emitter. Further, Judging from the recent trend of the requirement for operating various types of devices on low voltage, it is anticipated that the power supply voltage for portable wireless telephones and the like will be less than or equal to approx. 2.5 [V], thereby the above voltage drop will become a serious problem. For example, when the output signals of the circuit are amplified by use of a differential amplifier, the voltage drop occurs at the common emitter terminal of a pair of emitter-coupled transistors constructing the differential amplifier. When the power supply voltage is 2.5 [V] and if V.sub.BE of the transistor is approx. 0.7 [V], the potential of the common emitter terminal is set to approx. 0.1 to 0.4 [V] and it becomes difficult to operate a current source connected to the common emitter terminal. That is, in general mobile communication devices, the circuit following the orthogonal signal generation system will be disable for operating by the voltage drop.
In order to solve the above problem, it is possible to insert a capacitor between the output terminals of the circuit and the differential amplifier to cut off the DC component and shift the DC level of the input of differential amplifier by use of a level shifting circuit. However, with this construction, since it is necessary to form the level shifting circuit including the DC cut-off capacitor on an IC, the chip area and the lost of the power of the local signals are increased and the cost is raised and it is not preferable.
As described before, in the conventional orthogonal signal generation system, since the voltage (potential) of the output terminal is significantly lowered in comparison with a power supply voltage supplied to the system, it is not suitable for low voltage operation. Further, in the conventional orthogonal signal generation system, since it is required to attach a terminal resistor to the input side of a local signal, a sufficiently high voltage gain cannot be obtained.